Prevention of corrosion in urea synthesis



Patented Jan. 8, .1935

mmmms def Rom);

Wilmington, Del 'ass'ignors, by mesne assign- -ments, to E. L dn Pont de Nemimrs & com pany, Wilmington, Del., a corporation of Delaware NoiDi'awing Application November- 17,1930,

n "1 Serial No. 496,329-

Claims. (omen-e125) Thisinvention relates to'the synthesisof urea by reaction of ammonia and carbon dioxide or compounds thereof and more particularly to the, prevention of corrosion roi apparatus employed for handling urea synthesis reaction mixtures.

It is known that when ammonia andcarbon dioxide, or compounds of these substances with or without water, are heated in' a-closed system, a partial conversion to 'urea 1 takes "place. ',*This general reaction has formed the basisof numerous patents and discussions-in "theIiterature directed toward a process for the commercial synthesis of urea. The reaction mixtures, which for convenience will be referred was urea synthes'is melts may vary" widely inwcoinposition with varying proportions of reactants and/or with the extent to whichthje conversion to urea [has taken place. The synthesis melts-maycon- .tain, forfexamplavarious proportions of urea, ammonia, ammonium carbonates, ammonium ca'rbamat'e, and water. The-extremely corrosive nature of-urea synthesis meltsis known and a method has been proposed fol-the prevention of corrosion of chromium containing alloy steels in contact with these melts. In brief, this method consists in maintaining in -the synthesis melt a small concentration of dissolved copper, which may be added in the form of the metal or any suitable compound, copper carbonate, or copper oxide, for instance. The amount 'of'copper required is small, varying with the specific composition of the chromium alloy to be protected and being of the order of fromabout 0.03% to 0.3%, It is well known, however, that the vapor phase which may exist under some operating conditions over a urea synthesis melt is also corrosive though to a somewhat lesser extent than the melt proper.

We have found in practice that alloys which are somewhat resistant to attack in the liquid.

8 Ni, 1.5% Cu and 1.5% M; (0) a steel containing 18% Cr, 25% Ni and 3% Si. in view of the relatively expensive nature of the equipment required for urea synthesis it is high- However,

1y desirable to avoid corrosion entirely or at least tominimize it wherever possible. a

It is the object of the present invention to scribed. r

provide a method of. protection against vapor phase corrosion which will not only prolong the useful life .of materials normally somewhat res'is'tant to attack, but will also permit; the use in contact with the vapor phase, of chromium containing, alloy tacked. when unprotected.

Other objects andadvantages of the invention will be apparent ,as it is better. understoodby reference to the following specification in which steels which are rapidly atT- its details and preferred embodiments are deg Broadlyfspeaking the; invention consists in bathing or wetting, continuouslyor intermittently, the metallic surfaces exposedto, ,vapors above the urea synthesis melt,witha liquid containing a non-volatile corrosion inhibitor dissolv'edtherein, 1 Thus, for example, when "a vessel with inner surfacesof a' chromium containing alloy is employed in urea synthesis and theLproce ess is so operated that some'surfaces are exposed; to thevapor phase, corrosion of these lattersur faces is substantially inhibited by intermittently contacting them 'witliasolution containing small amounts of copper. The solution; employed may advantageously be theliquid phaseof the re action mass itself containing the necessary cop per for protection. Other points of introduction of copper solution may be utilized, however. For example, if the process is of the continuous type the copper required for protection in the liquid phase may be introduced as a solution of cuprous ammonium carbonate at such a point as to first come in contact with the walls exposed to the vapor phase and thereafter pass into the liquid phase. Alternatively, pieces, strips or sheets of copper may be suspended in the vapor phase and in contact with the wall of the vessel so that by vapors condensing on the copper, the latter will be partially dissolved and the condensate containing dissolved copper will run downover the apparatus wall.

Another method of carryingout the present invention consists in maintaining in the liquid phase a concentration of copper and circulating a small proportion ofthe melt by means of a pump which discharges through a pipe so disposed as to jet the liquid against the surfaces Y which it is desired to protect.

The foregoing methods are given as examples only and other methods of contacting the liquid with the walls of the vesselsemployed 'will' be readily apparent to those skilled in the art. v The frequency with which the walls-exposed to the vapor phase are wet with the" protecting liquid will depend in part upon, the means chosen, 3 to eifect the wetting. 'Ihe'fgreate'stdegree of:pro-.

tection will be obtained when the walls are constantly wetted. While it is preferable that the contacting periods should be so frequent as to approach continuity we have found that any wetting of the surfaces with a copper solution-as hereinbefore described is beneficial.

As previously stated the copper employed in the present process may be introduced either as metallic copper. or in the form of a suitable com pound, It is to be understood, therefore, that reference to copper broadly in the appended claims 'is to be taken as including copper whether introduced in the free state or in chemical combination.. V

The present invention is not restricted in its applicability tothe effecting of the synthesis reaction per se. If, for example, the separation of urea from unconverted raw materials and byproducts is effected by distillationin a vessel separate from the reaction vessel, corrosion during the distillationmay be prevented by employment of'the general process hereinbefore described.

It will be understood-that where reference is made in the specification or appended claims to the phrase ammonia and carbon dioxide, said phrase shall be construed as covering ammonia and carbon dioxide as such or as compounds containing the same.

Various changes may be effected in themethod.

and apparatus described without departing from thereof the invention or sacrificing any of the advantages We claim:

1. In the synthesis of urea involving the heating of ammonia and carbon dioxide in a metallic vessel the improvement which consists in protecting metallic surfaces exposed to the vapors above the synthesismelt by bathing saidsurfaces with a liquid containing dissolved copper.

2. In the synthesis of urea involving the heating of ammonia and carbon dioxide in a chromium steel vessel at elevated temperatures, the

-' improvement which consists in protecting metallic surfaces exposed to the vapors above the synthesis melt by bathing said surfaces with a liquid containing dissolved copper.

3. In the synthesisof urea involving the heating of ammonia and carbon dioxide in a metallic vessel, the improvement which consists in protecting metallic surfaces exposed to the vapors ing of ammonia and carbon dioxide in a chro-v mium steel vessel, at elevated temperatures, the

improvement'which consists in protecting metal l'ic surfaces exposed to the vapors above the synthesis melt by bathing said surfaces with a portion ofthe melt containing dissolved copper.

5. In the synthesis of urea involving the heat ing of ammonia and carbon dioxide in a chro- "3 mium steel vessel at elevated temperatures, the improvement which consists in protecting metallicsurfaces-exposed to the vapors above the syn-.

thesis melt by substantially continuously circulating in contact with said surfaces a portion of the melt containing dissolved copper.

, 6. In the synthesis of urea by reaction of ammonia and carbon dioxide in a chromium steel vessel 'at elevated temperatures, the improve,- ment which consists in continuously circulating into contact with metallic surfaces. exposed to vapors above the melt a portionof said meltcontaining dissolved copper, by introducing one of; the reactants in a gaseous state below the level of said melt.

1 HARRY. C. HETHERINGTON.

HAROLD W. DE ROPP. 

